Hello Friend,
You cannot fill registration form before completing your +2. To become a CS you have to pursue the course offered by The Institute of Company Secretaries of India (ICSI). There are three stages you have to clear, Foundation Program, Executive Program and Professional Program followed by 15 months of training. For more details refer the official website of ICSI: www.icsi.in
You can apply for the foundation course after completing your +2.